Soft Starter


17–1100 A and 200–690 V

Product movie

ISA-SL3 Heavy Duty Soft Starter with integrated bypass contacts

The new ISA-SL Appliance Range represents the product highlight for 2014 and impressively emphasises and evidences market leadership in softstarters. Apart from the purely external features, such as plug-connectable interface problem solutions throughout, the starter performance- and availability features were significantly improved. For this purpose, a combination of visual- and inductive techniques was introduced for the first time for the galvanic separation of the power circuits.

Advantages at a glance

  • USB interface for parametering and evaluation purposes
  • Unique motor protection package
  • Restart delay feature
  • Analog- or digital “IO” plug-connectable at any time
  • COM cards plug-connectable at any time
  • Emergency operation in 2 phase mode
  • 3 phase version
  • Integrated bypass contacts
  • Automatic frequency settings: 45 Hz – 65 Hz


For full view please move the table horizontally if necessary.
Starter Type kW at 400 A Width (mm) Height (mm) Depth (mm) Weight (kg)
ISA-SL3 17 7.5 119 245 146 3.2
ISA-SL3 31 15 119 245 146 3.2
ISA-SL3 44 22 119 245 146 3.2
ISA-SL3 58 30 128 275 208 5.2
ISA-SL3 72 37 128 275 208 5.2
ISA-SL3 85 45 128 275 208 5.2
ISA-SL3 105 55 172 368 234 10.9
ISA-SL3 145 75 172 368 234 10.9
ISA-SL3 170 90 172 368 234 10.9
ISA-SL3 210 110 366 644 278 37.0
ISA-SL3 240 132 366 644 278 37.0
ISA-SL3 310 160 366 644 278 37.0

Official Approvals

Current limit

The voltage increases linearly from the “Initial Voltage“ pre selected point. If the adjusted “Current Limit” is reached, the voltage will stop ramping and remain at its level until the current is reduced below the adjusted “Current Limit“ level. In that case ramp up time is prolonged until the current has reduced below “Current Limit” level 

Soft starter output voltage

Although the current is limited the motor provides suitable torque to accelerate the driven load and reach “End of acceleration”. If the motor torque does not exceed the load torque the motor will not start. In the so rare cases severe damages might occur to the motor and starter.

Motor start measured with an oscilloscope
(Ch1, voltage; Ch2-4 current L1 – L3)

The oscillogram shows, the soft starter providing a smooth and stepless current and voltage ramp to the motor. The voltage ramps up until “Current Limit” is reached. After the motor is at nominal speed, the current will decrease, to the nominal motor current.

Pump control

Our digital Soft Starters line of products ISA-D, ISA-DS and ISA-HD provide an advanced microprocessor based control algorithm, enabling the sophisticated pump control feature. The motor torque is continuously monitored to eliminate peak torque from stressing the motor, pump and pipe system. These soft starters provide 3 field selectable pump control algorithms.

Pump and special load control

Two major problems are associated with starting and stopping of pumps.

Over-pressure during starting

The sharp increase of torque towards the end of acceleration can cause high pressure and damage the pipe system. The pump control enables selection between three voltage ramp-up curves as well as torque curve to reduce peak torque. Current ramp is available for special loads.

Water hammer during stopping

During soft stop, when voltage is decreasing, motor torque may fall below load torque causing abrupt stalling instead of smoothly decreasing speed to zero. This creates the “water hammer” phenomenon (resulting in a loud noise and damage to the pipe system). The pump control algorithm enables selection between three voltage ramp-down curves or torque curve pre venting stall condition and eliminating water hammer.


  • Illuminated LCD with 2 lines of 16 characters each
  • Selectable languages: English, German, French or Spanish (aditional on request)
  • The LCD indicates the motor current, fault descriptions and statistical data.
  • The LEDs provide a quick status indication. (control voltage on, motor soft start, motor on, motor soft stop, motor stop, fault)
  • Easy settings by six clearly marked keys, user-friendly software with factory default parameters.

Communication RS 485

Modbus Profibus

RS 485 with MODBUS or PROFIBUS protocol enable:

  • Remote (start, stop, etc.)
  • Monitoring (motor current, faults, statistical data)
  • Remote parameter setting
  • Remote reset


Internal bypass

  • This reliable components bypass the power electronics after the motor is at speed.
  • In addition the starters provide an auxiliary contact, allowing signaling of a successful motor start.
    Attention! Many of our competitors call these auxiliary relays “a bypass contact”!
  • The bypass function reduces the heat dissipation and eliminates the EMC load of the system.